A vehicle seat is typically equipped with a seat belt to restrain the vehicle occupant from being thrown forward in case of a vehicle crash, and such a seat belt is sometimes provided with a retractor device equipped with an emergency locking retractor (which is referred to as ELR device hereinafter) for locking the winding spool of the seat belt in case of a sudden stop or a crash, however, without restraining the movement of the occupant under normal condition.
An ELR device typically detects a deceleration level indicative of a vehicle crash or a rapid pay-out of the seat belt before it locks up the winding spool in a very short period of time. Therefore, if the amount of the slack of the seat belt in the initial stage is excessive, there is a possibility that the seat belt may be inadequate to restrain the occupant to a necessary extent.
In view of such a problem, various devices for tightening the seat belt in case of a vehicle crash or preloader devices have been proposed:
A. Structures for winding a seat belt by driving the winding spool of the ELR device (refer to Japanese utility model laid out publication No. 54-169316);
B: Structures for pulling in the end of the seat belt opposite to the ELR device end by rotating a winding spool making use of the expansion of a propellant resulting from ignition and explosion thereof (refer to Japanese patent publication No. 53-21574); and
C: Structures for linearly pulling in a part of the seat belt paid out from the ELR device by coupling a clamp for gripping the seat belt to a piston which undergoes a linear displacement by the spring force of a spring or the like (refer to Japanese patent laid open publication No. 60-259553);
However, according to the structures of the categories A and B, if the seat belt is loosely wrapped around the winding spool of the ELR device, most of the power of the drive device is expended on tightly wrapping the seat belt around the winding spool and a sufficient restraint of the vehicle occupant may not be achieved.
According to the structures of the category C, a sufficient tension may be applied to the seat belt without regards to the state of the ELR device, but a relatively large displacement of moveable parts is required for linearly pulling the seat belt, and the size of the device tends to be excessively large in order to ensure a sufficient stroke of pulling the seat belt.
These ELR devices and preloaders are typically provided with a reversion preventing device consisting of a ratchet wheel and a ratchet pawl for permitting free unwinding and winding of the seat belt under normal condition and locking up the winding spool only in case of an emergency (Refer to Japanese patent publication No. 53-21574 and Japanese utility model publication No. 53-25943). However, according to such conventional arrangements, the ratchet pawl is urged into engagement with the ratchet wheel by a spring to prevent the rotation of the ratchet wheel in the reverse direction, and the reliability of such a reversion preventing device is inevitably much dependent on the elasticity of the spring which causes the necessary movement of the ratchet pawl. Therefore, the operation of the reversion preventing device is not entirely free from the chance of a failure if the ratchet pawl should be jammed or mechanically frozen.